转换为 swift 中的通用 class 类型

Question

My objective is to return the fetched results from the DB to any given type (model). For some reason I'm getting the error as "Use of undeclared type 'model'". What am I missing here? and how to achieve my objective.

func fetchData<T>(entity:String, model: T)-> T? {


    let request = NSFetchRequest<NSFetchRequestResult>(entityName:entity)
    request.returnsObjectsAsFaults = false

        do {

            let result = try managedObjectContext.fetch(request)
            if result.count > 0 {

                return result as model // Point that gets the error 

            }

        } catch {

          print("Failed to retrive data")
        }

    return nil

}

Answer 1

You want to use as and not is when casting and the type to cast to is T not model , or rather it is [T] since it is an array so the function declaration needs to be modified as well

func fetchDsata<T>(entity:String, model: T.Type)-> [T]? {
    let request = NSFetchRequest<NSFetchRequestResult>(entityName:entity)
    request.returnsObjectsAsFaults = false
        do {
            let result = try managedObjectContext.fetch(request)
            if result.count > 0 {
                return result as? [T]
            }
        } catch {
          print("Failed to retrive data")
        }
    return nil
}

Answer 2

There's no need to cast.

Based on Joakim's answer I recommend to make the method more generic and also throw to hand over the error to the caller.

Creating the fetch request with the generic type gets rid of the type cast

func fetchData<T : NSFetchRequestResult>(entity:String, model: T.Type) throws -> [T] {
    let request = NSFetchRequest<T>(entityName:entity)
    request.returnsObjectsAsFaults = false
    return try managedObjectContext.fetch(request)
}

And use it

do {
    let result = try fetchData(entity: “Foo”, model: Foo.self) 
    print(result)
} catch {
    print("Failed to retrive data")
}